1. Field of the Invention
The invention relates to a method of electrolytic tinning of a metal strip in a tinning line having a plurality of tinning tanks through which the strip moves in series while electrolysing current is passed, and having at at least one of the entry side and the exit side of the series of tinning tanks at least one buffer apparatus for storing a variable amount of the moving strip so that the strip speed in the tinning tanks can be temporarily different from the strip speed at the line entry or exit. Typically, such a buffer apparatus is a looping pit or a looping tower.
2. Description of the Prior Art
Tinning is generally used in the iron and steel industry to coat cold rolled steel strip with a layer of tin on one or both sides. In electrolytic tinning, a metal strip is fed through tinning tanks which are filled with an electrolyte. By passing a current through the electrolyte, the tin anodes, which hang in the electrolyte, go into solution at least partially and replace the material in solution which deposits on the metal strip which acts as cathode.
In such tinning lines there are entry and exit buffer apparatuses for the moving strip which are intended to achieve the result that disturbances in the entry and exit speeds of the strip are not reflected in corresponding changes of speed through the tinning tanks. Such changes in the throughput speed in the tanks are directly reflected in the tin deposit obtained on the metal strip. Further processing of the metal strip involves difficulties in that the tin deposit displays too great deviations from the desired value.
The tinned metal strip is mostly used for the production of tin cans, in which the deposited tin has two functions, namely first a lubricating property during the production phase of cans when the walls are stretched, and second a protective effect between the base material of the metal strip and the contents of the can. The tooling for the stretching of the walls is so adapted that a certain thickness of tin deposit is required for the proper working of the tools. Deviations in thickness can lead to the wall stretching machine seizing and even damage to the dies. These problems can be overcome by the buffer apparatuses at the entry and exit side of the tinning line so that the strip speed through the tinning tanks can be held constant in the face of the variation occurring in the entry and exit speeds of the metal strip. These variations stem from the necessity to connect a new metal strip at the entry side and to divide the strip at the exit side, both of which may be done on stationary strip e.g. at the entry side at a welding machine.
The buffer apparatuses used for this, e.g. looping pits or looping towers, require very high investment costs, which are strongly dependent upon the size of the buffer apparatus.
In the past adjustment of the total current through all the tinning tanks has been tried as a method of compensating to some extent for the variations in the speed of the metal strip through the tinning tanks, so that smaller buffer apparatuses could be used. However, it remains true that with this method the so-called over-plating and under-plating still occurs. This means that localised variations in the tin deposit occur, and these regions are not usable on quality grounds. This leads to the rejection of long pieces of metal strip.
Such variations in tin deposits appear to have a proportionally greater influence on the quality of the tinned product, as the desired quantity of deposited tin is smaller.
The market for tin cans has recently, with a view to cost reduction, seemed to have a tendency towards lower quantities of deposited tin. This is illustrated in that the usual quantity of tin deposited up to recently was 6 g/m.sup.2, and the forecast for the decade from 1990 is for a quantity of deposited tin of 1 g/m.sup.2.
The urge to develop a method in which a qualitatively good product can be made and guaranteed using a plant with buffer apparatuses of limited storage capacity, and therefore lower cost, is thus continually greater.